This invention relates to camshafts for four stroke internal combustion engines. More particularly it relates to camshafts that cause engine speed variable timing duration of combustion chamber valves and allow throttle-free engine load control by adjusting the valve timing.
Both petrol and diesel four stroke engines typically use a camshaft to control the opening and closing of the engine""s intake and exhaust valves. Normally the open period of the valves, usually referred to as the xe2x80x9cdurationxe2x80x9d or xe2x80x9cdwellxe2x80x9d, is fixed by the valve lobe shape or profile ground onto the lobe of the camshaft when it is manufactured. Normally, this profile cannot be varied without the physical replacement of the camshaft by another with a different profile ground onto its lobes.
On some engines that are described as having variable camshaft timing, the opening and closing points of the valves can be varied but the actual duration or dwell of the valve opening remains fixed. A conventional camshaft that provides a fixed amount of valve opening allows an engine to achieve maximum volumetric efficiency, and hence torque, at only one point in the engine""s revolution range. The torque falls off on either side of this point.
A camshaft arrangement which allows the valve opening duration to be varied so as to maximise the torque throughout the engine""s revolution range would be very desirable. This fact has long been realised by engine designers and much effort has been expended in the search of a mechanical variable duration system of valve timing. No successful system has been achieved for a mechanical continuously variable system of valve timing duration. Systems which are not continuously variable but operate on a two-stage principle, such as Honda""s VTEC system, have been adopted and are highly successful. Much effort is being spent on investigating hydraulic, pneumatic and solenoid systems of variable duration valve timing. Although the main advantage of a variable duration timing camshaft is to improve the torque spread of an engine it could be used to provide throttle-free control of the engine""s induction to minimise intake pumping losses and/or to achieve low exhaust emissions.
It has been proposed to use a camshaft having two closely spaced cam lobes in combination with a wider than normal follower, or tappet, that rides on both lobes simultaneously. A mechanism is provided so that the lobes can be aligned to give minimum duration or misaligned to give an increase in duration. If the misalignment does not exceed the angular distance of constant radius of the cam lobe""s nose, the follower xe2x80x9cseesxe2x80x9d the constant radius area as a continuous surface. The main deficiency of these devices is that the useable duration range is limited to twice, measured in degrees of rotation of the crankshaft, that of the angular span of the constant radius at the lobe""s nose. Any attempt to increase the duration past this angular distance results in the follower falling into the gap between the two lobe noses causing unacceptable noise and wear. There have been solutions proposed to this problem, but none have been commercially successful. There is a wide range of possible variations in lobe profiles, style of construction, even using lobes on two separate shafts, and methods of control and actuation of the duration change. However, none of these have provided a successful product.
It would be desirable to increase the upper duration limit to a much greater extent. This would lead to throttle free control. This would minimise pumping losses and would improve the overall fuel efficiency of a petrol engine to a level similar to that of a diesel engine. To achieve throttle free control of an engine it would need a maximum duration on the intake valve of something like 360 crankshaft degrees. The base duration on the intake valve of a typical road going engine is about 250 degrees with the valve opening 20 degrees before the top dead centre (BTDC) and closing 50 degrees after bottom dead centre (ABDC). This is normally expressed in conventional notation as 20-50/50-20 assuming the exhaust valve timing is also 250 degrees. Delaying the closing of the intake valve would cause some of the drawn in air and fuel mixture to be pushed back out of the cylinder before the intake valve closes thus resulting in a lesser amount of mixture to be combusted. By appropriately varying the amount of late closing of the valve, throttle free control of engine load could be achieved. For the minimum amount of power required that is, at idle, it would be expected that a closing point of about 160 degrees ABDC would be needed. As the intake valve opens at 20 degrees BTDC this would result in a total maximum duration of around 360 degrees being necessary, a range of 110 degrees. In conventional notation the valve timing at idle power would be written as 20-160/50-20. This is well beyond the capability of the earlier type of variable duration mechanism which has a typical range of 30 to 40 degrees.
This invention provides in one form a variable timing duration camshaft comprising an outer shaft and coaxial inner shaft wherein the shafts are adapted for relative slidable but not relative rotational engagement, and wherein the outer shaft has located in its outer surface valve control lobes, and wherein a valve control lobe modifying segment is located on the inner shaft and is slidingly fitted into a mating slot in the outer shaft and optionally in the valve control lobe whereby longitudinal axial movement of the inner shaft relative to the outershaft by a control means causes the modifying segment to move longitudinally and circumferentially relative to the valve control lobes thereby varying the duration.
In an alternative form the invention provides internal combustion engines having a variable timing duration camshaft as described above.